Here are more pictures. These are pictures of the restoration the PO did. I don't have any details about the carbs. Sorry about the quality, these are scanned Polaroids.
According to the PO it was a screamer....as long as it was going in a straight line. As soon as it went around a corner it would lose fuel and stall out. That's why he took the dual carbs off.
Those are Weber 40 DCNF carbs. Popular with the Bayless crowd years ago. Bayless swore by them against the IDF carbs, I think because they are so easy to tune. The problem is the float orientation is better suited for transverse mounted engines like in the X 1/9. If they are mounted on an in line engine like the spider they do just as you described when cornering.
Some guys have used a single 40 DCNF carb on an 1800 manifold for racing. It will require an adapter plate or relocation of the manifold studs. When mounted on the 1800 manifold the float is in the proper orientation. From what others have told me, a single 40DCNF carb if jetted correctly, can produce as much, if not more HP than dual 40's.
Its an absolute simple carb with very few parts and it's extremly light weight. I know there are also 40 DCN carbs and I'm not sure what's the difference, but legend has it the "F" in DCNF stands for Ferrari, as DCNF carbs were the standard carbs for Ferrari's.
VW guys also used DCNF carbs on bugs. I got a set off an old VW bug a few months ago for $20. The last 40 DCNf carbs I saw on Ebay went for $125 each.